Morever, the French can be a little unforgiving when a foreigner mangles their beloved language. However, efforts to speak in French are normally appreciated and tend to help make people more open and helpful!
About the language
French is France's unique official language and has been so since the eighteenth century. All schoolchildren are taught in the language and media are almost uniquely in French.
There are geographical variations in accent; this could make understanding someone from Marseille difficult if your teacher was from Calais. Take comfort from the fact your average native speaker might not understand 100% of what someone is saying if they have a strong regional accent. There are also some limited vocabulary differences between different parts of the country, mainly in colloquial or slang expressions.
Other languages in France
There are several minority languages clinging on for dear life in some areas (usual rural or border areas), including: Occitan, Catalan, Breton, Basque, Corsican, Flemish and Alsatian. It is highly unlikely you would ever be spoken to in any of these as they are only spoken within specific communities. It will be highly unusual to find a speaker of one of these languages not able to also speak standard French.
Do I really need French?
You might find a job without speaking French (especially in Paris), but you will have limited opportunities without a working knowledge of the language. If you do not have at least the ability to survive, get around and have basic conversations - as with living in any foreign country - you will probably not enjoy the experience as much as otherwise.